Such a diaphragm may equip a clutch, as described in French patent No. FR-A-2463874. In this document the clutch comprises a reaction plate fixed to the crankshaft of the engine and a cover fixed to the reaction plate. The diaphragm bears on the cover and on a pressure plate, connected in rotation to the cover but movable axially relative to the latter, for axial displacement of the pressure plate towards the reaction plate and to clamp between them a friction disc integrally in rotation with the input shaft of the gear box.
The diaphragm is moved by a clutch release bearing acting on the free ends of its fingers. The diaphragm may likewise equip a speed changer comprising, as described in the U.K. Patent No. GB-A-2140106, two pulleys, driving and driven respectively. Each of these comprises a hub and two annular flanges around the hub facing one another, one of which is fixed relative to the hub and the other being movable relative to the hub under the control of at least one diaphragm.
In the above document, in the case of each pulley, the diaphragm is covered by an elastic material and bears by means of its Belleville washer portion on the movable flange and by means of its central part on a bearing part fixed to the hub. The diaphragm drives the movable flange towards the other flange to clamp a belt. Operation of the speed changer is effected by varying the pressure in sealed chambers each delimited by the diaphragm, the movable flange and the bearing part.
In every case, during the operation of the clutch or of the speed changer, a flexing of the fingers of the diaphragm is produced which is greater than that of its Belleville washer portion, and there is therefore some dead travel.
To reduce this dead travel it has already been proposed in the German Patent Nos. DE-A-3307943 and DE-A-147543,7 to stiffen the fingers of the diaphragm by equipping their principal part with at least one fold made by making slits with defined shapes. Such arrangements are not entirely satisfactory since they complicate the fabrication of the fingers of the diaphragm and require special tooling.
To diminish the dead travel it is likewise possible to increase the thickness of the fingers. This arrangement is likewise unsatisfactory since, in addition to an onerous machining, it alters the Belleville washer portion of the diaphragm as a result of the discontinuity in thickness.
The object of the invention is to overcome these disadvantages while retaining the internal part of the fingers and the Belleville washer portion of the diaphragm, while obtaining other advantages and making use of the orifices in the diaphragm.